Combined DFT/CC and IR spectroscopic studies on carbon dioxide adsorption on the zeolite H-FER

Adsorption of carbon dioxide on H-FER zeolite (Si:Al = 8:1) was investigated by means of a combined methodology involving variable-temperature infrared spectroscopy and DFT/CC calculations on periodic zeolite models. The experimentally found value of adsorption enthalpy was ΔH⁰ = −30 kJ mol⁻¹. According to calculations, adsorption complexes on isolated Si(OH)Al Brønsted acid sites (single sites) involve an adsorption enthalpy in the range of −33 to −36 kJ mol⁻¹, about half of which is due to weak intermolecular interactions between CO₂ and the zeolite framework. Calculations show clearly the significant role played by weak intermolecular interactions; adsorption enthalpies calculated with standard GGA type exchange-correlation functionals are about 13 kJ mol⁻¹ underestimated with respect to experimental results. Good agreement was also found between calculated and experimentally observed stretching frequencies for these complexes. Calculations revealed that CO₂ adsorption complexes involving two neighbouring Brønsted acid sites (dual sites) can be formed, provided that the dual site has the required geometry. However, no clear evidence of CO₂ adsorption complexes on dual sites was experimentally found.